Refractory mold, method of making same and composition therefor



Jan. 7, 1958 J. P. BRADLEY EI'AL 2,818,619

REFRACTORY MOLD, METHOD OF MAKING SAME AND couposmon THEREFOR Filed Jan.51, 1957 REFRACTORY MOLD, METHGD OF MAKING SAME AND CUMPOMTHON THEREFORJames P. Eradley and Robert R. Bohrmann, Redford, Ind, assignors toGeneral Motors Corporation, Detroit, Mich, a corporation of DelawareApplication January 31, 1957, Serial No. 637,586

9 Claims. (Cl. 22-193) This invention relates to refractory molds forcasting metal and particularly to an improved investment material forforming such refractory molds. The present application is acontinuation-in-part of patent application Serial No. 324,706, which wasfiled on Decem "er 8, 1952, now abandoned.

in precision casting metallic articles by means of refractory molds itis frequently necessary that the casting surfaces of the molds possesshigh strength and exceptional smoothness. Thus, in investment molding itis usually desirable to form the principal refractory portion of themold of a hard material to which a coating on the invested pattern willtightly adhere, thereby pro-- viding the mold with a smooth castingsurface upon removal of the destructible pattern. The mold must havesufficient strength and rigidity so as to Withstand the pressure of themolten casting metal. It is important, moreover, to use an investmentmaterial which not only has the necessary porosity, strength andrigidity, but which also possesses surface and setting characteristicspermitting it to bond tightly to the coating on the pattern afterinvestment.

Accordingly, a principal object of the present invention is to providean improved investment composition for forming refractory molds to beused in precision casting operations. A further object of this inventionis to provide a refractory mold and a process for forming the samewherein the investment material tightly adheres to the coating on thedestructible pattern upon investment. Moreover, upon setting, thisinvestment material is sufiiciently strong and rigid to aidin'preventing warping, cracking or rippling of the coating layer underthe localized pressure caused by the molten casting metal.

The above objects are obtained in accordance with the present inventionby the provision of an investment material consisting of a mixture of anethyl silicate binder and a dry mix or grog comprising a pulverized fireclay, a fine refractory filler material, and an accelerator for thebinder. Borax glass is preferably included in the mixture to increaseits high-temperature strength.

Other objects and advantages of this invention will more fully appearfrom the following detailed description of the invention in conjunctionwith the accompanying drawing, which contains a somewhat schematicsectional view of a destructible pattern invested in a refractory moldformed in accordance with the invention.

Referring more particularly to the drawing, a pattern 10 of a turbinebucket to be cast is shown invested in a refractory mold 12 within ametallic container or flask 14 positioned on a base plate 22;. Thepattern in is prefer-' ably formed of a low fusing substance, such aswax or a thermoplastic material, or any other vaporizable, fusible,combustible or otherwise destructible material. However, plasticpatterns are preferably employed in order to provide optimum resultswith the refractory mold embodying the present invention. Among theplastic patterns which have been found to be satisfactory are thoseformed 2,8 l 3 ,6 l Patented Jan. 7, 1-958 of polystyrene, althoughother thermoplastic pattern materials, such as resinous polymerizedderivatives of acrylic acid and resinous polymerized derivatives ofmethacrylic acid, may be used.

The pattern may be initially cast under pressure in a conventionalmanner. A gating portion 116 having a pourlug-basin part it; at itsouter end is attached to the pattern, the portions 16 and i8 usuallybeing formed of a destructible material similar to that of the pattern.After the pattern is formed, it is preferably cleaned with an alcoholsolution and air dried prior to application of the coating solution.Before the pattern is invested within the mold, it is coated with anappropriate coating material which is to form the casting surface 2% ofthe refractory mold. This coating material may be comprised of anaqueous dispersion of conventional finely comminuted refractorymaterials, a binder, and defoaming and wetting agents, an air-settingsilicate cement being preferably used as the binder. Gelatine and acidsmay also be included in the coating mixture. This type of coating isdisclosed in our co-pending patent application Serial No. 320,736, filedNovember 15, 1952, now Patent No. 2,752,257.

Coating of the pattern is preferably accomplished by dipping the patternin the coating solution. Although in some instances the coating may alsobe applied by spraying or painting it on the pattern or in any othersuitable manner, dipping is preferred because it assures more uniformcoating of all the pattern surfaces and is the simplest method ofapplication.

The dip coat slurry is preferably kept in constant motion by stirringmeans except during the actual dipping operation. However, the mixingaction should not be such as to unnecessarily introduce air into theslurry. The destructible pattern used for forming turbine buckets orother precision cast parts is immersed in the dip coat slurry,preferably to within /2 to inches of the end of the gating portion ofthe pattern. Care should be exercised in immersing the pattern in theslurry to prevent air entrapment on the pattern. Normally the dip coatsolution is retained at room temperature during the dipping operationbecause excessive heat can result in distortion of the plastic pattern.The excess coating material is permitted to drain off prior tosubsequent treatment and investment.

After the pattern has been completely coated with the dip coat slurry,it may be sanded or stucco-e to provide a rough surface on the coatingthus insuringv greater adhesion between the principal refractory portion12 of the mold and the dip cost 20 on the pattern. This sanding may beaccomplished by merely screening or otherwise applying silica or othersuitable refractory materials in known manner to the outer coatedsurface of the destrucible pattern. When all the molding surfaces of thepattern have been effectively covered with sand, the pattern should beair dried.

Following the formation of the pattern, the investment material 12 isformed about the pattern and sprue, the latter being permitted to extendthrough the wall of the resultant refractory mold so as to permit theescape of the destructible pattern material and to form an ingate forthe fluid casting metal. This main refractory mold may be formed aboutthe pattern in any suitable manner, but the following procedure providesexcellent results. The base plate 22 is preferably first sprayed orotherwise coated with molten wax so as to form a thin film of wax overits upper surface. Before the wax is completely solidified the patternto be invested is positioned on the plate 22 with the gate and pouringbasin portions 16 and 18 thereof extending downwardly and seated firmlyin the wax film. The sleeve or flask i4 is then placed around thepattern and pressed lightly into the Wax layer. In

3 order to completely seal the flask 14 to the plate 22, it ispreferable to again spray molten wax over the outer surfaces of theseparts at their junction. The resultant assembly should be allowed to setfor several minutes to permit the wax to thoroughly solidify.

After the refractory mixture, which will be hereinafter described indetail, has been mixed with a proper amount of the liquid binder, it ispoured into the sleeve or flask 14, which is preferably vibrated duringthis pouring opera tion, and the mold is then allowed to set.

When the mold body has solidified or set to a sulficient extent, thebase plate 22 is removed from beneath the mold and heat is applied tomelt the pattern. It is necessary to apply sufiicient heat to raise themold temperature above the fusing point of the material, thus permittingthe molten pattern to escape through the gate and sprue opening in themold formed by the pattern portions 16 and 18. In this manner the dipcoat which had covered the pattern tightly adheres to the remainder ofthe mold and provides the casting cavity with a smooth coating. It isalso possible to vaporize the pattern, if a vaporizable material isused, by heating the mold rapidly to a high temperature.

After removal of the pattern from the mold in the foregoing manner, themold is burned out to remove substantially all the volatile matter. Themold is then preferably preheated to the desired temperature, and themolten casting metal is poured or otherwise introduced into the moldcavity formed by the pattern. In the majority of instances it isnecessary to pour the casting metal while the mold is still hot. Afterthe molten metal has been poured and the casting has solidified, therefractory mold body 12 and the adhering coating 20 may be broken topermit the removal of the casing. As a result of using the improvedrefractory mold formed in accordance with the present invention, thefinished casting possesses excellent surface smoothness and detail andrequires few finishing operations.

The investment material used to form the body or principal refractoryportion 12 of the mold embodying the invention consists of a dry mix orgrog to which an ethyl silicate binder is added. This dry mix comprisesmajor proportions of a pulverized fire clay, such as dead burned fireclay or fire brick and a finely comminuted refractory material and aminor proportion of an accelerator or gelation agent for the binder.Magnesium oxide is the setting accelerator preferably employed, butmagnesium carbonate, calcium carbonate, sodium carbonate, and otheralkaline oxides or carbonates may be used. In order to improve thehigh-temperature bond of the resultant mold, borax glass is preferablyalso included in the mix. The binder for the grog consists of an aqueoussolution of con densed ethyl silicate, alcohol and an acid. Denaturedethanol of approximately 190 proof spirit is the alcohol preferablyemployed, while concentrated hydrochloric acid is the acid which we havefound most desirable to add to the binder solution.

Accordingly, an investment dry mix or grog which provides excellentresults is one comprising, by weight, approximately 65% to 90% of thefinely ground fire clay or brick, 9% to 34% silica flour or other finelycomminuted refractory material, and 0.15% to 1.5% of an accelerator orgelation agent, such as magnesium oxide, for the binder. In order toprovide optimum results, the fineness of this mix should be between 90and 100 A. F. A. The accelerator content which is preferred for mostapplications is between 0.2% and 1%, the greater the amount ofaccelerator added, the lesser the gel time. When borax glass is includedin the grog to provide the resultant mold with improved high-temperaturebond, it should be present in an amount not in excess of 2% by weight.Moreover, quantities of b-orax glass as small as 0.1% have proved to bebeneficial in improving the hightemperature strength of the mix. Atpresent we prefer to use a grog containing about 0.3% to 1% borax glass.

The ethyl silicate type of binder used in accordance with the inventioncomprises, by weight, approximately 35% to 60% condensed ethyl silicate,35% to 60% alcohol, 0.1% to 0.4% concentrated hydrochloric acid and 5%to 13% Water. Depending on the particular application, the ratio, on aweight basis, of the dry mix or grog to the ethyl silicate binder mayvary from about 2.5 to 4.5. When the above investment dry mix and thebinder are mixed in the proper ratios, the resultant investment materialgenerally comprises, by weight, approximately 43% to 74% pulverized fireclay or brick, 6% to 25% silica flour, 0.1% to 1.2% magnesium oxide, 6%to 17% condensed ethyl silicate, 6% to 17% alcohol, 0.02% to 0.1%hydrochloric acid and 0.9% to 3.7% water. A sufiicient amount ofadditional water may be added during mixing of the grog and binder toraise the water content to as high as 7%. If borax glass is included inthe mix ture, in accordance with the preferred embodiment of theinvention, it preferably constitutes between 0.1% and 1.6% of the finalinvestment material. For optimum result-s the fire clay content shouldbe between 55% and 70% of the weight of the entire mixture, this amountbeing equivalent to approximately 70% to 87% of the dry mix or grog.

The finely ground fire clay functions as the refractory base of theinvestment mixture and must be selected so that it has desirableexpansion characteristics. This material should also be of uniformquality and composition and reasonably free of foreign matter. Calmo isan example of a pulverized dead burned fire clay. Various mixtures ofrelatively coarse and fine Calmo fire clays may be used, these claysvarying in A. F. A. finenesses from approximately 20 to 120.

Silica flour is aded to increase the fines in the re fractory base andto eliminate voids in the backing of the mold. Other refractory powders,such as zirconium silicate or zirconium oxide flour, may be used forparticular applications, however. Generally it is desirable to use asilica flour which is fine enough to permit at least 99% of it to passthrough a 140 mesh screen. The magnesium oxide is used in powdered formand may consist of either heavy or light magnesium oxide, or mixtures ofthese oxides. It is preferable that this material be of sufficientfineness so that at least 90% of it will pass through a 200 mesh screen.Likewise, the borax glass, which functions as a secondaryhigh-temperature bonding agent in the investment mix, should be added inpowder form, preferably of approximately 90 to 110 mesh.

The condensed ethyl silicate in the investment binder, of course, is asource of silica for the reaction in which ethyl silicate and waterreact to form silica gel and alcohol. Upon drying, silica is theultimate binder for the investment. Alcohol is included to producemutual solubility of ethyl silicate and Water since these latterconstituents are immiscible in the absence of the alcohol. Thehydrochloric acid is necessary for pH control and to regulate the speedof the aforementioned reaction and the subsequent mold gelation.

It is preferable to use a condensed ethyl silicate having not less than25% available silica as SiO An example of such an ethyl silicate is oneconsisting of approximately tetraethyl orthosilicate and 15%polysilicates. Satisfactory commercially available products of this composition frequently have 0.1% maximum acidity as hydrochloric acid, aflash point (open cup) of approximately F, and a specific gravitybetween 0.920 and 0.950 at 20 C. Hence a typical example of the ethylsilicate solution preferably used is a condensed ethyl silicate whichcontains, by volume, approximately 50% ethyl silicate, 0.1% hydrochloricacid, and the balance alcohol and water. It will be appreciated,however, that any hydrolyzed or condensed ethyl silicate solution may besatisfactorily used so long as the ethyl silicate content is sufficientto provide the proper bonding properties.

With respect to the permissible upper limit of the concentration of theethyl silicate solution, it is desirable that condensing conditionsexist. Accordingly, to assure these conditions, we have found itfeasible to use a solution which contains only enough water to provide asuflicient amount of hydrolysis to obtain the above satisfactory bondingand strengthening effects, or to otherwise use ethyl silicate underhydrolyzing conditions. Thus, we have obtained best results by the useof a mixture containing hydrolyzed or condensed ethyl silicate solutionwith the ethyl silicate content between 25% and 75 by volume.

In preparing the binder solution, the water and acid are first mixedtogether, and the alcohol and condensed ethyl silicate are subsequentlyadded. The solution is then stirred and permitted to set for severalhours prior to use. A preferred method of mixing the grog and liquidbinder consists of adding approximately two-thirds of the binder and theadditional water, preferably about 2% of the weight of the binder, to arotating batch mixer. The dry mix is then slowly added to the liquid andthe slurry mixed for approximately ten minutes.

The above-described high-strength refractory mold for investment moldingis particularly adapted for use in coating articles having curvedsurfaces, such as turbine buckets, because it tightly adheres to thecoating on the destructible patterns and prevents distortion of thiscoating. Furthermore, the resultant mold does not react with nickel-basealloys, a material used in cast turbine buckets, and hence has noadverse effect on the surface qualities of such buckets.

While the present invention has been described by means of certainspecific examples, it is to be understood that the scope of theinvention is not to be limited thereby except as defined in thefollowing claims.

We claim:

1. A mold composition consisting essentially of a mixture of an ethylsilicate solution and a grog comprising, by weight, approximately 65% to90% of a pulverized fire clay, 9% to 34% of a finely comminutedrefractory material, a small but effective amount of borax glass not inexcess of 2%, and 0.15 to 1.5% of a setting accelerator for the ethylsilicate solution.

2. A mold composition consisting essentially of a mixture of a dry grogcomprising, by weight, approximately 65% to 90% of a pulverized fireclay, 9% to 34% of a finely comminuted refractory material, 0.15% to1.5% of a setting accelerator for ethyl silicate and 0.1% to 2% powderedborax glass, and an ethyl silicate binder solution comprising, byweight, about 35% to 60% condensed ethyl silicate, 35% to 60% alcohol,0.1% to 0.4% of an acid, and 5% to 13% water.

3. An investment composition for a refractory mold consisting of amixture of an ethyl silicate solution and a grog comprising, by weight,approximately 65 to 90% of a finely pulverized fire clay, 9% to 34%silica flour, 0.2% to 1% of a setting accelerator for ethyl silicate anda small but effective amount of borax glass not in excess of 2%, theratio, by weight, of the grog to the ethyl silicate solution beingbetween 2.5 to 1 and 4.5 to 1.

4. An investment composition for a refractory mold consistingessentially, by weight, of approximately 70% to 87% of a finely groundburned fire clay, 9% to 34% silica flour, 0.2% to 1% magnesium oxide and0.1% to 2% borax glass, and an ethyl silicate binder solutioncomprising, by weight, about 35% to 60% condensed ethyl silicate, 35% to60% alcohol, 0.1% to 0.4% concentrated hydrochloric acid and 5% to 13%water.

5. An investment composition for a refractory mold consistingessentially, by weight, of approximately 55% to of a finely pulverizeddead burned fire clay, 6% to 25 silica flour, 0.1% to 1.2% magnesiumoxide, 0.1% to 1.6% powdered borax glass, 6% to 17% denatured ethylalcohol, 0.02% to 0.1% concentrated hydrochloric acid, 0.9% to 3.7%water, and 6% to 17% condensed ethyl silicate solution, the ethylsilicate content in said solution being between 25% and by volume.

6. A refractory mold having a body portion resulting from setting of amixture of an ethyl silicate solution and a grog consisting essentially,by weight, of approximately 65 to of a finely pulverized fire clay, 9%to 34% silica flour, 0.2% to 1% of a setting accelerator for ethylsilicate and a small but effective amount of borax glass not in excessof 2%, the ratio, by weight, of the grog and ethyl silicate solutionbeing between 2.5 and 4.5.

7. A refractory mold comprising an investment body portion formed of theresidue of a mixture consisting, by weight, essentially of approximately43% to 74% of a pulverized fire clay, 6% to 25% of a finely comminutedrefractory material, 0.1% to 1.6% powdered borax glass, 6% to 17%condensed ethyl silicate, 6% to 17% alcohol, 0.02% to 0.1% of an acid,0.9% to 7% Water, and 0.1% to 1.2% of a setting accelerator for thecondensed ethyl silicate.

8. A method of forming a refractory mold which comprises coating adestructible pattern with a refractory coating mixture, investing saidcoated pattern in a refractory molding mix consisting essentially of agrog comprising, by weight, approximately 65 to 90% of a finely groundfire clay, 9% to 34% silica flour, a small but effective amount of boraxglass not in excess of 2% and 0.15 to 1.5% of a setting accelerator forethyl silicate, and a binder for said grog comprising a solution ofapproximately 35% to 60% condensed ethyl silicate, 35% to 60% alcohol,0.1% to 0.4% of an acid and 5% to 13% water, and thereafter eliminatingthe pattern from said mold, whereby said coating adheres to therefractory mold.

9. The process of forming a refractory mold having a smooth castingsurface, said process comprising applying a refractory coating to afusible pattern, drying said coating, thereafter investing said coatedpattern in an investment material comprising, by weight, approximately43% to 74% of a finely pulverized dead burned fire brick, 6% to 25%silica flour, 0.1% to 1.2% magnesium oxide, a small but effective amountof borax glass not in excess of 1.6%, 6% to 17% condensed ethylsilicate, 6% to 17% ethyl alcohol, 0.02% to 0.1% concentratedhydrochloric acid and 0.9% to 7% water, setting the investment material,melting and removing the pattern from the formed mold, whereby thecoating tightly adheres to the walls of the casting cavity of theinvestment material, and thereafter treating said mold at a temperaturesuflicient to remove substantially all of the volatile matter therefrom.

References Cited in the file of this patent UNITED STATES PATENTS2,027,932 Ray Jan. 14, 1936 2,333,430 Lee et al Nov. 2, 1943 2,441,695Feagin et al. May 18, 1948 2,568,364 Duesbury et al. Sept. 18, 1951FOREIGN PATENTS 74 Great Britain of 1906 585,665 Great Britain Feb. 18,1947 641,187 Great Britain Aug. 9, 1950

1. A MOLD COMPOSITION CONSISTING ESSENTIALLY OF A MIXTURE OF AN ETHYLSILICATE SOLUTION AND A GROG COMPRISING, BY WEIGHT, APPROXIMATELY 65% TO90% OF A PULVERIZED FIRE CLAY, 9% TO 34% OF A FINELY COMMINUTEDREFRACTORY MATERIAL, A SMALL BUT EFFECTIVE AMOUNT OF BORAX GLASS NOT INEXCESS OF 2%, AND 0.15% TO 1.5% OF A SETTING ACCELERATOR FOR THE ETHYLSILICATE SOLUTION.